There are applications where hydrocarbons are to be transferred between floating structures, such as between a production vessel that produces and stores hydrocarbons from an undersea reservoir and a buoy for offloading the stored hydrocarbons at regular intervals to a tanker that is moored to the buoy. The hydrocarbons can be transferred through a pipeline that extends in the sea between the structures and that is connected in a pipe joint at each structure. One problem encountered with such a system is that there is repeated stressing of each pipe end due to pitch, roll and heave of the corresponding floating structure. Such repeated stressing, especially in the wave action zone, can result in fatigue failure of the pipe end and of a corresponding pipe connector on the floating structure. Ways to construct a fatigue resistant midwater pipe and ways to minimize such stressing at minimum costs would be of value.
U.S. Pat. No. 6,779,949 shows a catenary or U-shaped steel midwater pipe where the pipeline ends are placed entirely below the wave turbulent zone. The pipe ends are connected to the floating structure, or floater with flexible hoses in the wave active zone. U.S. Pat. No. 6,769,376 shows a midwater system which includes multiple steel pipe sections with clamped fixed spacers at the pipe section ends and flexible spacers in between, which allows for a relative movement of the pipes to each other. These patents include either a upper flexible part or a spacer means.
Patent application GB2335723 shows a riser decoupling system with a weight-carrying chain or cable part between the floater and the end of the steel midwater pipe. In this way relative movement between the buoy and the end of a subsea pipeline is accommodated by a suspended member in the form of a chain, rope or cable. In that patent the fluid path between the end of the pipeline and the buoy includes a flex hose. Other systems for decoupling the motion of the surface floater from a steel midwater pipes by creating a distance between the steel midwater pipe end and the floater, are shown in patent publications U.S. Pat. No. 6,109,989 and US20030084961.
Patent application WO03062043 shows a special design for a deepwater buoy which at its lower part is connected to a steel horizontal transfer duct via a flex joint. The sections of the steel transfer pipe are welded together and are subject to large fatigue loads as it is placed in the wave active zone. The design of the buoy is such that it reduces fatigue loads of the mooring lines and the horizontal transfer duct; the buoy is therefore made slender and relatively long such that the horizontal fluid transfer duct extends below the wave active zone. The fluid duct is therefore less subject to fatigue loads due to the shape of the buoy and the fact that it is placed under the wave active zone, so that a welded midwater pipe arrangement can be used without the danger of (fatigue) cracks being introduced to the welded area of the midwater pipe.